1. Field of the Invention
The present invention relates to a loud speaker to be used for various acoustic apparatuses, and a method for producing the same.
2. Description of the Related Art
FIG. 1 is a half cross-sectional view showing a configuration for a typical loud speaker 20. FIG. 2 is an exploded perspective view showing details of the loud speaker 20. The same constituent elements are indicated by the same reference numerals in FIGS. 1 and 2.
As shown in FIGS. 1 and 2, the loud speaker 20 includes a lower plate 3 integral with a center pole 2, a magnet ring 4 provided on a bottom portion of the lower plate 3 so as to surround the center pole 2, and an upper plate 5 provided on an upper face of the magnet ring 4. The lower plate 3, the magnet ring 4, and the upper plate 5 are coupled to one another to constitute a magnet circuit 1.
On an upper face of the upper plate 5, an inner periphery of the frame 6 is coupled. A gasket 7 and an outer periphery of a diaphragm 8 are attached to an outer periphery of the frame 6 using an adhesive. A voice coil 9 is coupled to an inner periphery of the diaphragm 8.
A middle portion of the voice coil 9 is supported by an inner periphery of the damper 10, an outer periphery of the damper 10 being supported by the frame 6. A lower portion of the voice coil 9 is inserted into a magnetic gap 11 formed between the center pole 2 of the lower frame 3 and the upper frame 5 (which are included in the magnetic circuit 1) without being eccentric. Moreover, a dust cap 12 for preventing dust from entering the magnetic circuit 1 is provided on the upper side of a central portion of the diaphragm 8.
It is preferable that the material constituting the diaphragm 8 has such properties as high elasticity, low density, and high internal loss for the following reasons.
The high-frequency range resonance frequency of the diaphragm 8 increases as a specific elasticity E/.rho.(where E represents the elasticity modulus and .rho. represents the density) of the material constituting the diaphragm 8 increases, that is, as the elasticity modulus E increases and as the density .rho. decrease. Such a loud speaker is capable of reproducing sounds in a higher frequency range and therefore realizing a broader reproduction range.
Moreover, the diaphragm 8 achieves a flatter frequency characteristic curve and a lower distortion rate as the internal loss of its material increases.
In view of the above, a principal material used for the diaphragm 8 of the conventional loud speaker 20 is paper which is composed mainly of natural pulp such as wood pulp. This is because paper has an appropriate elasticity modulus and internal loss as well as low density, and therefore provides advantages that a diaphragm composed of a synthetic resin or a complex thereof cannot attain.
On the other hand, the voice coil 9 is required to withstand a large input signal applied thereto. In order for a loud speaker to have good resistance for such a large input, the voice coil 9 is required to have an increased inflammability and heat resistance for the following reasons.
When an input signal is applied to the voice coil 9, an electric current flows in a coil (not shown in FIG. 1 or 2) of the voice coil 9 so as to generate Joule's heat. The Joule's heat increases as the level of the input signal increases, thereby drastically raising the temperature of the voice coil 9. As a result, a bobbin (not shown in FIG. 1 or 2) around which the coil is wound may be burnt, or varnish which is used to couple the coil to the bobbin may deteriorate through softening, causing the coil to fall off the bobbin.
FIG. 3 shows an exemplary configuration for a conventional voice coil 9 designed so as to overcome the above-mentioned problem. The voice coil 9 includes a bobbin 13 composed of a strip of a metal foil, e.g., aluminum, bent into a cylindrical shape. Kraft paper 14 is wound, for reinforcement and insulation, around an outer periphery of the voice coil 9 where a coil 15 is not wound. The bobbin 13 is obtained by winding the voice coil 9 on a portion of the bobbin 13 where the kraft paper 14 is not wound. In this configuration, the coil 15 is directly wound on the metal foil constituting the bobbin 13, so that the metal foil functions to radiate the heat generated in the coil 15, thereby preventing elevation of temperature.
Recently, there has been a trend for using metals such as aluminum or organic foams for the material of the diaphragm 8, instead of the above-mentioned paper. However, organic foams have low elasticity and cannot attain sufficient characteristics. On the other hand, a metal diaphragm has only a small internal loss and the weight thereof is large. Therefore, these substitute materials for paper are not optimum materials for diaphragms of loud speakers for use in acoustic apparatuses.
There have been developed diaphragms for loud speakers made of materials consisting of inorganic fibers and/or organic synthetic fibers mixed with paper so as to improve the elasticity of the paper. However, the expected effect of improving the elasticity has not been attained.
Furthermore, paper diaphragms tend to absorb, and therefore are generally susceptible to, moisture. For example, paper diaphragms are not appropriate for such applications as loud speakers to be attached on the doors of automobiles, which require a particularly good water-proofness. In order to solve this problem, diaphragms for loud speakers requiring a high degree of water-proofness have typically been produced by adhering water repellent on pulp fibers during fabrication, or impregnating the fabricated paper diaphragm with a synthetic resin solution so as to provide the paper with water-proof properties.
Very recently, however, the loud speakers to be attached on the doors of automobiles have particularly been required to be sufficiently resistant against surfactants included in detergents for washing automobiles, e.g., car shampoos. The above-mentioned method of adhering water repellent on pulp fibers or impregnating the fabricated paper diaphragm with a synthetic resin solution cannot attain sufficient resistance against such surfactants.
One solution to this problem has been proposed, according to which a water-proof synthetic resin film is laminated onto a surface of a paper diaphragm after the fabrication thereof. However, this creates a new problem of the need for specific jigs and equipment for attaching the synthetic resin film onto the paper diaphragm.
In order to overcome the above-mentioned problems, Japanese Patent Publication No. 57-40718 describes a diaphragm produced by using a material including a principal material of short fibers, such as polyethylene, polypropylene, nylon, and polyacrylonitrile, or synthetic pulp obtained by fibrillating these fibers, and a subordinate material of fibers such as inorganic fibers, organic synthetic fibers, or natural fibers mixed in the principal material, subjecting the material to a paper-fabrication process, and melting the resultant complex synthetic pulp so as to mold it into a desired shape. This diaphragm has excellent environmental characteristics such as water-proofness. However, the diaphragm also has the three following problems.
First, it is difficult to reduce the density of the obtained diaphragm because high-density inorganic fibers, e.g., carbon fibers, alumina fibers, and glass fibers, are mixed into the principal material in order to improve the elasticity of the molded product.
Second, the synthetic pulp used for the above-mentioned diaphragm has relatively short fiber lengths and therefore has low freeness. As a result, the fabrication process takes a long time.
Third, the synthetic pulp used for the above-mentioned diaphragm has a relatively high beating degree, and has relatively short fiber lengths, so that it is difficult to obtain a bulky product after a percolation process. Moreover, since the synthetic pulp is melted during the drying-molding process, the obtained molded product has a film-like shape, so that it is difficult to increase the thickness of the molded product and to adequately reduce the density and increase the internal loss thereof.
On the other hand, in the conventional voice coil 9 shown in FIG. 3, the metal foil used for the bobbin 13, which is incorporated with a view to improving the heat resistance of the voice coil 9, has a large weight, thereby deteriorating the performance of the loud speaker. Moreover, since metals are good electrical conductors, the use of a metal foil for the bobbin 13 may cause a short-circuiting of the coil 15.
Alternatively, a sheet composed of heat-resistant chemical fibers, such as paper composed of aromatic polyamide fibers, e.g., aramid paper or NOMEX paper (manufactured by Du Pont Ltd.) is occasionally used for the bobbin 13 of the voice coil 9. However, such paper slightly absorbs moisture. As a result, when the temperature of the voice coil 9 rapidly increases, the moisture absorbed in the paper is gasified so that swelling may occur in a portion of the bobbin 13 where the coil 15 is wound. Furthermore, it is difficult for the bobbin 13 as described above to be completely severed. Thus, a portion of one or more of the aromatic polyamide fibers may be left at the severed surface. These fibers may also remain in a plumous state on the surface of the sheet. In either case, such portions of the aromatic polyamide fibers can cause extraordinary noises during the operation of the loud speaker, thus deteriorating the quality of the loud speaker.
Furthermore, as described above in connection with the diaphragm 8, loud speakers to be attached on the doors of automobiles are required to be particularly water-proof, so that the voice coil 9 is also required to have an improved water-proofness as well as the diaphragm 8.